Method for producing a preferential oxidation catalyst

ABSTRACT

This is a hydrogen generation process for use with fuel cells which includes a preferential oxidation step to reduce the concentration of carbon monoxide. The preferential oxidation step includes contacting a fuel stream comprising hydrogen and carbon monoxide in the presence of an oxygen at a preferential oxidation temperature of between about 70° and about 160° C. with preferential oxidation catalyst for reducing the concentration of carbon monoxide to produce a treated fuel gas stream comprising less than about 50 ppm-vol carbon monoxide. The preferential oxidation catalyst comprises ruthenium metal dispersed on a shaped alumina carrier, wherein at least 60 percent of the ruthenium metal is present in a band extending from the surface towards the center and having a width of about 50 percent of the distance from the surface to the center of the shaped alumina carrier. Superior performance at low preferential oxidation temperatures below 130° C. was observed when the band comprised 50 percent of the alumina carrier and contained at least 60 percent of the ruthenium metal. The preferential oxidation catalyst may be employed to reduce carbon monoxide in fuel gas streams supplied to fuel cells wherein carbon monoxide will poison the active noble metal membrane and higher preferential oxidation temperatures may reduce the hydrogen content of the gas stream.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.09/583,618 filed May 31, 2000, now U.S. Pat. No. 6,409,939 the contentsof which are hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a method of producing a catalyst andthe use of the catalyst in the selective oxidation of carbon monoxide.More particularly, the present invention relates to a method ofproducing a catalyst and the use of the catalyst in a process for thecatalytic preferential oxidation of carbon monoxide in a fuel gas streamprior to the use of the fuel gas stream in a fuel cell.

BACKGROUND OF THE INVENTION

Fuel cells are in principle batteries in which the energy obtained fromthe reaction of a fuel stream comprising hydrogen and oxygen isconverted directly into electrical energy. The present inventiondescribes the preparation of catalysts for preparation of the fuel gasstream for use in fuel cells, in particular for PEM (polymer electrodemembrane) fuel cells. This type of fuel cell is becoming increasinglyimportant, due to its high energy density and robust structure, for usein the vehicle industry, i.e. for providing electro-traction in motorvehicles.

The advantages of a vehicle powered by fuel cells are the very lowemissions and the high degree of efficiency of the total system comparedwith conventional internal combustion engines. When hydrogen is themajor component in the fuel gas, the primary emission product of theconversion in the fuel cell is water. The water is produced on thecathode side of the fuel cell. The vehicle is then a so-called ZEV (zeroemission vehicle). The use of hydrogen in fuel cells requires thathydrogen be available on the anode side of the fuel cell membrane toactually generate power. The source of the hydrogen can be stationary ormobile. Stationary sources of hydrogen will require a distribution anddispensing system like motor gasoline. Mobile sources for hydrogen willinclude on-board hydrogen generators for the conversion of hydrocarbonfuels to hydrogen. However, hydrogen presents many handling anddistribution problems which will not be resolved before the fuel cellpowered vehicles reach the market. The infrastructure for the widespreaddistribution of hydrogen is still too expensive at the moment and thereare other problems with the storage and refueling of vehicles. For thisreason, the alternative, producing hydrogen directly on board thevehicle by reforming hydrocarbon fuels or oxygenated fuels is growing inimportance. For example, methanol can be stored in a fuel tank of thevehicle and on demand converted by a steam reforming process at 200° to300° C. to a hydrogen-rich fuel gas with carbon dioxide and carbonmonoxide as secondary constituents. After converting the carbon monoxideby a shift reaction, preferential oxidation (prefox) or anotherpurification process, this fuel gas, or reformate gas is supplieddirectly to the anode side of the PEM fuel cell. Theoretically, thereformate gas consists of 75 volume percent hydrogen and 25 volumepercent carbon dioxide. In practice, however, the reformate gas alsowill contain nitrogen, oxygen and, depending on the degree of purity,varying amounts of carbon monoxide (up to 1 volume percent).

The PEM fuel cell comprises layers of catalyst comprising platinum andplatinum alloys on the anode and cathode sides of PEM fuel cells. Thesecatalyst layers consist of fine, noble metal particles which aredeposited onto a conductive support material (generally carbon black orgraphite). The concentration of noble metal is between 10 and 40 weightpercent and the proportion of conductive support material is thusbetween 60 and 90 weight percent. The crystallite size of the particles,determined by X-ray diffraction (XRD), is about 2 to 10 nm. Traditionalplatinum catalysts are very sensitive to poisoning by carbon monoxide;therefore the CO content of the fuel gas must be lowered to <100 ppm inorder to prevent power loss in the fuel cells resulting from poisoningof the anode catalyst. Because the PEM fuel cell operates at arelatively low operating temperature of between 70° and about 100° C.,the catalyst is especially sensitive to CO poisoning.

Processes for the production of synthesis gas are well known andgenerally comprise steam reforming, autothermal reforming, non-catalyticpartial oxidation of light hydrocarbons or non-catalytic partialoxidation of any hydrocarbons. Of these methods, steam reforming isgenerally used to produce synthesis gas for conversion into ammonia ormethanol. In such a process, molecules of hydrocarbons are broken downto produce a hydrogen-rich gas stream. A paper titled “Will DevelopingCountries Spur Fuel Cell Surge?” by Rajindar Singh, which appeared inthe March 1999 issue of Chemical Engineering Progress, page 59-66,presents a discussion of the developments of the fuel cell and methodsfor producing hydrogen for use with fuel cells and highlights one hybridprocess which combines partial oxidation and steam reforming in a singlereaction zone as disclosed in U.S. Pat. No. 4,522,894 B1 which is herebyincorporated by reference.

U.S. Pat. No. 5,922,487 B1 discloses an anode electrocatalyst for a fuelcell which depresses the poisoning of the noble metal fuel cellmembrane. The anode electrocatalyst comprises an alloy essentiallyconsisting of at least one of tin, germanium, and molybdenum, and one ormore noble metals selected from platinum, palladium, and ruthenium.

U.S. Pat. No. 6,007,934 B1 is concerned with the preparation ofsupported catalysts based on platinum and ruthenium disposed on theanode side of a PEM fuel cell which have a high resistance to poisoningby carbon monoxide. Carbon monoxide concentrations of more than 100 ppmin the reformate gas should be possible to employ in the fuel gas passedto the fuel cell without a noticeable drop in performance of the PEMfuel cell.

U.S. Pat. No. 6,010,675 B1 discloses a method and apparatus for removingcarbon monoxide from a fuel gas prior to use of the fuel gas in a fuelcell for the production of electric power. Catalysts for purifyinghydrogen by selective oxidation of carbon monoxide using aluminasupported platinum are disclosed in an article entitled “PurifyingHydrogen by . . . Selective Oxidation of Carbon Monoxide” by Marion L.Brown, Jr. et al, Industrial and Engineering Chemistry, Vol. 52, No. 10,October 1960, pp. 841-844. U.S. Pat. No. 6,010,675 B1 discloses theproblem of using a conventional preferential oxidation catalyst systemin a hydrogen generator or fuel processor for producing a fuel gasstream for use in a fuel cell. The above mentioned article at page 842-3indicated that the selective removal of carbon monoxide was feasibleonly within a certain temperature zone for all known selective oxidationcatalysts with or without variation of the oxygen concentration, belowwhich the oxygen reaction falls off. The critical temperature range forthe effective preferential oxidation was identified as being above 130°C. (266° F.) and below 160° C. (320° F.). U.S. Pat. No. 6,010,675 B1 andthe above mentioned article are hereby incorporated by reference. Thearticle stated that this narrow range of selectivity applied to a widerange of precious metal catalysts supported on aluminum oxide.

An article entitled “Advanced PEFC Development For Fuel Cell PoweredVehicles”, by Shigeyuki Kawatsu, published in the Journal of PowerSources, Volume 71 (1998), pages 150-155, discloses that a rutheniumcatalyst on alumina was found to be useful for reducing the carbonmonoxide concentrations of reformed gas from methanol reforming over awider operating temperature range than platinum based oxidationcatalysts. Significant carbon monoxide conversion activity between about100° and about 160° C. was disclosed.

EP 0955351 A1 discloses a CO-selective oxidation catalyst having metalsincluding platinum and ruthenium disposed on an alumina carrier. Thecatalyst preparations included ruthenium metals on alumina pellets withruthenium metal loadings up to 1.0 weight percent. EP 0955351 A1discloses that the active temperature range for ruthenium was about 160°to 180° C., and only when platinum was either alloyed with the rutheniumor when platinum was included on the alumina carrier was a desiredactive temperature below 160° C. achieved.

In order to achieve a balance between the reforming reaction zone andthe high and low temperature water gas shift reaction zones of fuelprocessors, others have attempted to dispose these reaction zone inintimate thermal contact to minimize overall energy use. The addition ofa preferential or selective oxidation zone to such an integrated systemwherein the preferential oxidation catalyst requires effective operatingconditions above the outlet conditions of the low temperature water gasshift reaction and above the temperature of the fuel cell operationcreates a difficult engineering problem. On the reaction side, theincreased temperature may result in hydrogen loss, and on theengineering side, heating the effluent form the water gas shift reactionzone to the favorable temperature range of the selective oxidationreaction and then cooling the selective oxidation effluent requiresincreased mechanical complexity, and increased equipment cost.

An object of the present invention is to provide preferential oxidationcatalysts which have an improved conversion of carbon monoxide. It is anobjective of the present invention to provide a preferential oxidationcatalyst which operates effectively at conditions which are morefavorable in reducing the carbon monoxide concentration in the fuel gasin fuel cell systems. It is an objective of the present invention toprovide and, in particular, to achieve effluent concentrations of carbonmonoxide of less than about 50 ppm-vol. Another object of the presentinvention is to provide a method of producing stable catalysts suitablefor the selective conversion of carbon monoxide while maintaining areasonably high selectivity to the production of carbon dioxide withoutregeneration.

SUMMARY OF THE INVENTION

The present invention relates to a process for the production of a fuelgas for use in a fuel cell which is sensitive, and in fact, is poisonedby the presence of carbon monoxide in the fuel gas. The fuel gas is ahydrogen-rich stream resulting from the conversion of a hydrocarbon oran oxygenate to produce a synthesis gas which may contain up to about 2mole percent carbon monoxide. Previously known catalysts forpurification of hydrogen streams required more severe conditions thanare present in fuel processors or than could be accommodated in acompact fuel processor and fuel cell arrangements. The problem solved bythe present invention is a more active preferential catalyst which canreduce the concentration of carbon monoxide in the prefox effluent toless than about 50 ppm-vol at preferential oxidation conditionsconsistent with the operation of the fuel cell. More specifically, thepreferential oxidation catalyst, or prefox catalyst, of the presentinvention effectively reduces the carbon monoxide in a hydrogen-richfuel gas to concentration levels below 50 ppm-vol, at a wide range ofpreferential temperatures including temperatures below 180° C., andparticularly below 160° C. Preferably, the wide range of preferentialoxidation temperatures includes temperatures between about 70° and about130° C. The catalyst of the present invention was found to provideeffective reduction of carbon monoxide from hydrogen-rich streams. Itwas surprisingly discovered that by using the method of the presentinvention to disperse active metal on the surface of a catalyst carrier,a stable and active preferential oxidation catalyst is obtained.

In one embodiment, the present invention relates to a process for thegeneration of a hydrogen-rich fuel gas stream for use in a fuel cell forthe generation of electric power. The process comprising passing a feedstream comprising a hydrocarbon or an oxygenate to a fuel processor. Thefuel processor comprises an integrated reforming and water gas shiftconversion zone to produce a fuel stream. The fuel stream compriseshydrogen, carbon monoxide, carbon dioxide, and water. The fuel stream atan effective oxidation temperature of between about 70° and about 160°C. and in the presence of an oxygen-containing stream is passed to apreferential oxidation zone. The preferential oxidation zone contains apreferential oxidation catalyst to produce the hydrogen-rich fuel gasstream comprising less than about 50 ppm-vol carbon monoxide. Thepreferential oxidation catalyst comprises ruthenium metal dispersed on ashaped alumina carrier, at least 60 percent of the ruthenium metal beingpresent in a band extending from the surface towards the center andhaving a width of about 50 percent of the distance from the surface tothe center of the shaped alumina carrier. The hydrogen-rich fuel gasstream is passed to a fuel cell for the generation of electric power andelectric power is withdrawn.

In another embodiment, the present invention relates to a method forpreparing a preferential oxidation catalyst to reduce the concentrationof carbon monoxide in a hydrogen-rich fuel gas stream produced by a fuelprocessor for a fuel cell to generate electric power. The method forpreparing the preferential oxidation-catalyst composition comprisescontacting a shaped alumina carrier with a source of ruthenium metalcomprising ruthenium nitrosyl nitrate at a pH of between about 1.0 andabout 4.5 to provide a ruthenium-containing composition. Theruthenium-containing composition has a ruthenium metal content ofbetween about 0.5 and about 3 weight percent of the catalyst asruthenium metal dispersed on a shaped alumina carrier, at least 60percent of the ruthenium metal being present in a band extending fromthe surface towards the center and having a width of about 50 percent ofthe distance from the surface to the center of the shaped aluminacarrier. The ruthenium containing composition is reduced to provide thepreferential oxidation catalyst.

In a further embodiment, the present invention relates to a preferentialoxidation process for the conversion of carbon monoxide. This processcomprises passing a fuel stream comprising hydrogen, carbon monoxide,carbon dioxide and water in the presence of an oxygen-containing streamat oxidation conditions including a preferential oxidation temperaturebetween about 70° and about 160° C. to a reaction zone. The reactionzone contains a preferential oxidation catalyst which comprisesruthenium metal dispersed on a shaped alumina carrier, at least 60percent of the ruthenium metal being present in a band extending fromthe surface towards the center and having a width of about 50 percent ofthe distance from the surface to the center of the shaped aluminacarrier. A treated fuel stream comprising less than about 50 ppm-volcarbon monoxide is withdrawn from the preferential oxidation process.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE depicts ruthenium distribution in the ruthenium-containingpreferential catalyst of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Generally, the feedstock to a preferential oxidation process using thecatalyst of the present invention will comprise hydrogen, nitrogen,carbon monoxide, carbon dioxide, water, and light hydrocarbons and istypically derived from a combination of reforming and water gas shiftreaction step. Some sulfur compounds including hydrogen sulfide andmercaptans may be present. The removal of sulfur from the hydrocarbonfeedstock may be accomplished prior to the reforming and water gas shiftsteps by any conventional means including adsorption, chemisorption, andcatalytic desulfurization. For compact fuel processors used with fuelcells, chemisorption with a material such as zinc oxide is preferred.The desulfurization operation will generally take place at effectiveconditions including a desulfurization pressure of between about 100 andabout 1000 kPa. Preferably, the desulfurization operation is carried outat a desulfurization pressure of between 200 and 300 kPa. Preferably,the desulfurization operation is carried out at a desulfurizationtemperature less than about 300° C., and more preferably, thedesulfurization operation is carried out at a desulfurizationtemperature between about 50° and about 300° C. Preferably, theconcentration of sulfur in the desulfurized feedstock will be less thanabout 10 ppm-mol, and more preferably, the concentration of sulfur inthe desulfurized feedstock will be less than about 1 ppm-mol. Theremoval of carbon monoxide from the fuel gas sent to the fuel cell is ofkey importance.

Carbon monoxide poisoning of the fuel cell membranes will result in thereduction of the electrical output of the fuel cell. The catalyst of thepresent invention is effective in reducing carbon monoxide fromfeedstock concentrations ranging from about 100 to about 10,000 ppm-volto provide a treated product stream comprising less than about 50 ppmcarbon monoxide. Preferably, the treated product stream comprisesbetween about 1 and about 50 ppm-vol carbon monoxide. More preferably,the treated product stream comprises less than about 10 ppm-vol carbonmonoxide.

An essential feature of the present invention involves the use of acatalytic composite comprising a combination of catalytically effectiveamounts of a ruthenium component with a porous carrier material whereinthe ruthenium is dispersed on the surface of the porous carriermaterial. Catalytically effective amounts of ruthenium metal at aneffective preferential oxidation temperatures between about 70° andabout 160° C. range between about 0.5 and about 10 weight percentruthenium metal on the catalyst of the present invention. Moreparticularly, the preferential oxidation catalyst of the presentinvention will contain ruthenium in amounts ranging from about 0.5 toabout 5 weight percent ruthenium metal, and most particularly, thecatalyst of the present invention will contain ruthenium in amountsbetween about 1 and about 3 weight percent ruthenium metal. Preferably,the preferential oxidation reaction will be conducted at temperaturesbelow 160° C., and even below 140° C. Preferably, the preferentialoxidation conditions include a preferential oxidation temperaturebetween 70° and about 130° C. and a preferential oxidation pressure ofbetween about 7 and about 250 kPa (1 to about 30 psia).

Considering first the porous carrier material utilized in the presentinvention, it is preferred that the material be a porous, adsorptive,high surface area support having a surface area of about 25 to about 500m²/gm. The porous carrier material should be relatively refractory tothe conditions utilized in the preferential oxidation process, and it isintended to include within the scope of the present invention carriermaterials which have traditionally been utilized in hydrocarbonconversion catalysts such as: (1) activated carbon, coke, or charcoal;(2) silica or silica gel, silicon carbide, clays, and silicatesincluding those synthetically prepared and naturally occurring, whichmay or may not be acid treated; (3) ceramics, porcelain, crushedfirebrick, bauxite; (4) refractory inorganic oxides such as alumina,titanium dioxide, zirconium dioxide, chromium oxide, zinc oxide,magnesia, thoria, boria, silica-alumina, silica-magnesia,chromia-alumina, alumina-boria, silica-zirconia, etc.; (5) crystallinealuminosilicates such as naturally occurring or synthetically preparedmordenite and/or faujasite, either in the hydrogen form or in a formwhich has been treated with multivalent cations; and, (6) combinationsof these groups. The preferred porous carrier materials for use in thepresent invention are refractory inorganic oxides, with best resultsobtained with an alumina carrier material. Suitable alumina materialsare the crystalline aluminas known as the gamma-, eta-, andtheta-alumina with gamma- or eta-alumina giving best results. Inaddition, in some embodiments, the alumina carrier material may containminor proportions of other well known refractory inorganic oxides suchas silica, zirconia, magnesia, etc.; however, the preferred support issubstantially pure gamma- or eta-alumina. Preferred carrier materialshave an apparent bulk density of about 0.3 to about 0.7 gm/cc, anaverage pore diameter of about 20 to 3000 angstroms, a pore volume ofabout 0. 1 to about 2.5 ml/gm, and a surface area of about 100 to about500 m²/gm.

The preferred alumina carrier material may be prepared in any suitablemanner and may be synthetically prepared or naturally occurring.Whatever type of alumina is employed, it may be activated prior to useby one or more treatments including drying, calcination, steaming, etc.,and it may be in a form known as activated alumina, activated alumina ofcommerce, porous alumina, alumina gel, etc. For example, the aluminacarrier may be prepared by adding a suitable alkaline reagent, such asammonium hydroxide to a salt of aluminum such as aluminum chloride,aluminum nitrate, etc., in an amount to form an aluminum hydroxide gelwhich upon drying and calcining is converted to alumina. The aluminacarrier may be shaped or formed in any desired shape such as spheres,pills, cakes, extrudates, powders, granules, etc., and utilized in anydesired size. By the term “shaped alumina carrier”, it is meant toinclude shaped particles, spheres, cylinders, and irregularly shapedparticles. The shaped alumina carrier will have a geometric surface andif the shaped alumina carrier is a particle, it will have a center. Thealumina carrier may also be disposed as a coating on a monolith, and insuch cases, the alumina carrier will have an outer surface and an innersurface where the coating is disposed on the monolith. In the presentinvention, the ruthenium metal is not uniformly dispersed in the aluminacarrier, but is dispersed in a band extending from the surface towardthe center in particles and from the surface toward the monolith whenthe alumina carrier is disposed on a monolith. For the purpose ofillustrating present invention, a particularly preferred form of aluminais the sphere; and alumina spheres may be continuously manufactured bythe well-known oil drop method which comprises forming an aluminahydrosol by any of the techniques taught in the art and preferably byreacting aluminum metal with hydrochloric acid, combining the resultinghydrosol with a suitable gelling agent and dropping the resultantmixture into an oil bath maintained at elevated temperatures. Thedroplets of the mixture remain in the oil bath until they set and formhydrogel spheres. The spheres are then continuously withdrawn from theoil bath and typically subjected to specific aging treatments in oil andan ammoniacal solution to further improve their physicalcharacteristics. The resulting aged and gelled particles are then washedand dried at a relatively low temperature of about 300° F. (149° C.) toabout 400° F. (204° C.) and subjected to a calcination procedure at atemperature of about 850° F. (454° C.) to about 1300° F. (704° C.) for aperiod of about 1 to about 20 hours. It is also a good practice tosubject the calcined particles to a high temperature steam treatment inorder to remove as much as possible of undesired acidic components. Thismanufacturing procedure effects conversion of the alumina hydrogel tothe corresponding crystalline gamma-alumina. See the teachings of U.S.Pat. No. 2,620,314 B1 for additional details.

A material characteristic of typical amorphous refractory oxide supportmaterials employed in the preparation of catalysts is shown in U.S. Pat.No. 5,494,568 B1 and hereby incorporated by reference. The pore sizedistribution of this porous carrier or refractory oxide support materialis shown in Table 1 based on the pore diameter of samples determined bywell-known mercury porosymmetry analysis.

TABLE 1 Carrier Pore Size Distribution Pore Diameter, Percent of Total ÅPore Volume 30-91 4.41 101-152 25.30 162-192 5.08 202-253 1.04 262-2930.25 303-404 0.33 456-556 0 607-758 0

An unusual feature of the present invention is the character of theruthenium metal on the porous refractory oxide carrier. It wassurprisingly discovered that dispersing ruthenium metal in a bandextending from the surface toward the center of a shaped alumina carriermaterial resulted in significant improvement in terms of CO conversionover the same shaped alumina carrier with a uniform ruthenium metaldistribution throughout the alumina carrier. In comparative testing ofruthenium-containing alumina based preferential oxidation catalysts, itwas discovered that the more the ruthenium was concentrated in a bandnear the surface of the shaped alumina carrier, the more active and morestable was the preferential oxidation catalyst.

To prepare the preferred catalyst, the support material is contactedwith one or more precursors of catalytically active ruthenium metal toprepare ruthenium-containing catalyst. The ruthenium containing catalystis then dried in air at a temperature of about 100° to about 120° C. fora period of from about 15 minutes to about 4 hours. The dried catalystcomposition is purged with nitrogen and then reduced at a reductiontemperature of about 200° to 500° C. in a reducing atmosphere for aperiod of about 0.5 to about 2 hours. Optionally, catalyst preparationsteps can include an oxidation step at an elevated temperature followedby a reduction step at an elevated temperature. It is preferred to usean oxidation temperature below about 500° C., and more preferably, it ispreferred to use an oxidation temperature between about 200 and about500° C.

In the preparation of the preferential oxidation catalyst, a number ofruthenium compounds such as ruthenium nitrate, ruthenium chloride,ruthenium iodide, ruthenium chloride, and the like are useful forcontacting the shaped alumina carrier material to create the catalyst ofthe present invention. Preferred compounds include ruthenium nitrosylnitrate, ruthenium carbonate, etc. It is most preferred to employruthenium nitrosyl nitrate solution in amounts to provide an effectiveruthenium content of about 0.5 up to about 10 weight percent rutheniummetal of the catalyst. More preferably, the finished preferentialoxidation catalyst contains an effective amount of ruthenium equal toabout 0.5 to about 5 weight percent of the catalyst as ruthenium metal,and most preferably, the finished preferential oxidation catalystcontains an effective amount of ruthenium equal to about 1.5 to about 3weight percent of the catalyst as ruthenium metal.

The preparation of the preferential oxidation catalyst of the presentinvention was surprisingly influenced by controlling the pH of theruthenium contacting solution. When no adjustment to the pH of thecontacting solution was made, the ruthenium metal was disperseduniformly throughout the shaped alumina carrier. Significant improvementin the CO conversion occurred when the pH of the ruthenium contactingsolution was adjusted to a value greater than 0.0, and more particularlyover a range from about 1.0 to about 4.5. As the pH of the rutheniumcontacting solution was increased from about 1.0 to about 4.5, theruthenium metal on the catalyst was dispersed preferentially in a bandextending from the surface of the shaped alumina carrier toward thecenter. For example, in a spherically shaped alumina carrier particle,having a geometric surface and a center, there was a band extending fromthe surface to about 50 percent of distance toward the center of theshaped alumina carrier, and that band contained about 67 percent of theruthenium metal dispersed on the catalyst, when the ruthenium contactingsolution was adjusted to a pH of about 1.0. Essentially all of theruthenium dispersed on the catalyst was dispersed on the catalyst in aband extending from the surface of the shaped alumina carrier toward thecenter when the ruthenium contacting solution was adjusted to a pHgreater than about 1.0. As the pH of the ruthenium contacting solutionused to contact the shaped alumina carrier was increased to about 4.5,essentially all, or at least about 98 percent, of the ruthenium metalwas dispersed in a band extending from the surface to about 10 percentof the distance toward the center of the shaped alumina carrier. Theseobservations were based on SCM and EDX analysis of the shaped aluminacarrier.

The preferred ruthenium catalyst of the invention provides greatersustainable carbon monoxide conversion for an equivalent amount ofruthenium than comparative ruthenium-containing catalysts. Otherphysical properties of the final catalyst of the present inventioninclude a total pore volume of about 0.1 to about 2.5 cc/gram, andpreferably about 0.3 to about 1.8 cc/gram, and an apparent bulk densitybetween about 0.2 and about 0.7, and preferably a density between about0.2 and about 0.4 grams/cc.

The catalyst of the present invention may be employed in a finishingstep in the production of a fuel gas for a fuel cell to reduce thecarbon monoxide in this fuel gas and thereby minimize damage or extendthe life of PEM fuel cells. Catalyst of the present invention also maybe employed in any of several conversion processes including steamreforming and water gas shift processes.

Optionally, the catalyst may also contain other, additional componentsor mixtures thereof which act alone or in concert as catalyst modifiersto improve catalyst activity, selectivity or stability. Some well-knowncatalyst modifiers include antimony, arsenic, beryllium, bismuth,cadmium, calcium, chromium, cobalt, copper, gallium, gold, indium, iron,lithium, magnesium, manganese, molybdenum, nickel, potassium, rhenium,scandium, silver, sodium, tantalum, thallium, titanium, tungsten,uranium, zinc, and zirconium. These additional components may be addedin any suitable manner to the carrier material during or after itspreparation or they may be added in any suitable manner to the catalyticcomposite either before, while, or after other catalytic components areincorporated.

The invention is further illustrated by the following examples which areillustrative of specific modes of practicing the invention and are notintended as limiting the scope of the invention defined in the claims.

EXAMPLES Example I (pH 4.5)

A 25-cc aliquot of the alumina support as characterized in Table 1 asspheres or extrudates weighing 12.9 g was introduced into a rotaryevaporator. A stock solution of ammonium hydroxide (^(˜)28 percent NH₃)was diluted with deionized water to a 50:50 mixture and then graduallyadded to a solution of 26.6 g of ruthenium (III) nitrosyl nitratesolution containing 1.5 percent ruthenium until a pH of 4.56 wasreached. This solution was then added to the alumina support in ajacketed rotary evaporator. The support and solution were kept at roomtemperature in the rotating apparatus for about 15 minutes. Steam wasintroduced into the jacket of the rotary evaporator for a periodsufficiently long so as to allow free-flow of the impregnated catalystspheres (or extrudates). This period lasted for approximately 2 to 4hours.

The impregnated catalyst was placed in a quartz tube surrounded by aceramic-lined tube furnace and a flow of air was introduced to the tubeat ambient temperature and at a flow rate of 6 L/min for a period ofabout 15 minutes. The temperature was raised at 5° C./min to about 110°C. and held there for about 2 hours with the same constant air flow. Theair flow was shut off and a nitrogen purge was commenced at a flow rateof 1 L/min at 110° C. for 30 minutes. The nitrogen flow was terminatedand a hydrogen flow was introduced over the impregnated catalyst at arate of 3 L/min. The temperature was raised at a rate of 5° C./min to500° C. while continuing hydrogen flow. The temperature was thenmaintained at 500° C. for a period of 1 hour. Hydrogen flow was stoppedand nitrogen introduced as the temperature was slowly brought down toroom temperature.

Example II (pH 3.0)

A 25-cc aliquot of the alumina support as characterized in Table 1 asspheres or extrudates weighing 12.9 g was introduced into a rotaryevaporator. A stock solution of ammonium hydroxide (^(˜)28 percent NH₃)was diluted with deionized water to a 50:50 mixture and then graduallyadded to a solution of 26.6 g of ruthenium (III) nitrosyl nitratesolution containing 1.5 percent ruthenium until a pH of 3.0 was reached.This solution was then added to the alumina support in a jacketed rotaryevaporator. The support and solution were kept at room temperature inthe rotating apparatus for about 15 minutes. Steam was introduced intothe jacket of the rotary evaporator for a period sufficiently long so asto allow free-flow of the impregnated catalyst spheres (or extrudates).This period lasted for approximately 2 to 4 hours.

The impregnated catalyst was placed in a quartz tube surrounded by aceramic-lined tube furnace and a flow of air was introduced to the tubeat ambient temperature and at a flow rate of 6 L/min for a period ofabout 15 minutes. The temperature was raised at 5° C./min to about 110°C. and held there for about 2 hours with the same constant air flow. Theair flow was shut off and a nitrogen purge was commenced at a flow rateof 1 L/min at 1 10° C. for 30 minutes. The nitrogen flow was terminatedand a hydrogen flow was introduced over the impregnated catalyst at arate of 3 L/min. The temperature was raised at a rate of 5° C./min to500° C. while continuing hydrogen flow. The temperature was thenmaintained at 500° C. for a period of 1 hour. Hydrogen flow was stoppedand nitrogen introduced as the temperature was slowly brought down toroom temperature.

Example III (pH 2.0)

A 25-cc aliquot of the alumina support as characterized in Table 1 asspheres or extrudates weighing 12.9 g was introduced into a rotaryevaporator. A stock solution of ammonium hydroxide (^(˜)28 percent NH₃)was diluted with deionized water to a 50:50 mixture and then graduallyadded to a solution of 26.6 g of ruthenium (III) nitrosyl nitratesolution containing 1.5 percent ruthenium until a pH of 2.0 was reached.This solution was then added to the alumina support in a jacketed rotaryevaporator. The support and solution were kept at room temperature inthe rotating apparatus for about 15 minutes. Steam was introduced intothe jacket of the rotary evaporator for a period sufficiently long so asto allow free-flow of the impregnated catalyst spheres (or extrudates).This period lasted for approximately 2 to 4 hours.

The impregnated catalyst was placed in a quartz tube surrounded by aceramic-lined tube furnace and a flow of air was introduced to the tubeat ambient temperature and at a flow rate of 6 L/min for a period ofabout 15 minutes. The temperature was raised at 5° C./min to about 110°C. and held there for about 2 hours with the same constant air flow. Theair flow was shut off and a nitrogen purge was commenced at a flow rateof 1 L/min at 110° C. for 30 minutes. The nitrogen flow was terminatedand a hydrogen flow was introduced over the impregnated catalyst at arate of 3 L/min. The temperature was raised at a rate of 5° C./min to500° C. while continuing hydrogen flow. The temperature was thenmaintained at 500° C. for a period of 1 hour. Hydrogen flow was stoppedand nitrogen introduced as the temperature was slowly brought down toroom temperature.

Example IV (pH 1.0)

A 25-cc aliquot of the alumina support as characterized in Table 1 asspheres or extrudates weighing 12.9 g was introduced into a rotaryevaporator. A stock solution of ammonium hydroxide (^(˜)28 percent NH₃)was diluted with deionized water to a 50:50 mixture and then graduallyadded to a solution of 26.6 g of ruthenium (III) nitrosyl nitratesolution containing 1.5 percent ruthenium until a pH of 1.0 was reached.This solution was then added to the alumina support in a jacketed rotaryevaporator. The support and solution were kept at room temperature inthe rotating apparatus for about 15 minutes. Steam was introduced intothe jacket of the rotary evaporator for a period sufficiently long so asto allow free-flow of the impregnated catalyst spheres (or extrudates).This period lasted for approximately 2 to 4 hours.

The impregnated catalyst was placed in a quartz tube surrounded by aceramic-lined tube furnace and a flow of air was introduced to the tubeat ambient temperature and at a flow rate of 6 L/min for a period ofabout 15 minutes. The temperature was raised at 5° C./min to about 1 10°C. and held there for about 2 hours with the same constant air flow. Theair flow was shut off and a nitrogen purge was commenced at a flow rateof 1 L/min at 110° C. for 30 minutes. The nitrogen flow was terminatedand a hydrogen flow was introduced over the impregnated catalyst at arate of 3 L/min. The temperature was raised at a rate of 5° C./min to500° C. while continuing hydrogen flow. The temperature was thenmaintained at 500° C. for a period of 1 hour. Hydrogen flow was stoppedand nitrogen introduced as the temperature was slowly brought down toroom temperature.

Example V

A 400-cc aliquot of the alumina support as characterized in Table 1 asspheres or extrudates weighing 120 g was introduced into a rotaryevaporator. A solution composed of 247 g of ruthenium (III) nitrosylnitrate solution containing 1.5 percent ruthenium was diluted with 153 gof deionized water and added to the alumina support in a jacketed rotaryevaporator. No adjustment of pH was performed. The support and solutionwere kept at room temperature in the rotating apparatus for about 15minutes. Steam was introduced into the jacket of the rotary evaporatorfor a period sufficiently long so as to allow free-flow of theimpregnated catalyst spheres (or extrudates). This period lasted forapproximately 4 hours.

The impregnated catalyst was placed in a quartz tube surrounded by aceramic-lined tube furnace and a flow of air was introduced to the tubeat ambient temperature and at a flow rate of 6 L/min for a period ofabout 15 minutes. The temperature was raised at 5° C./min to about 110°C. and held there for about 2 hours with the same constant air flow. Theair flow was shut off and a nitrogen purge was commenced at a flow rateof 1 L/min at 110° C. for 30 minutes. The nitrogen flow was terminatedand a hydrogen flow was introduced over the impregnated catalyst at arate of 3 L/min. The temperature was raised at a rate of 5° C./min to500° C. while continuing hydrogen flow. The temperature was thenmaintained at 500° C. for a period of 1 hour. Hydrogen flow was stoppedand nitrogen introduced as the temperature was slowly brought down toroom temperature.

Example VI (Testing Example)

Samples of the catalyst (3 ml) of Examples I-IV with nominally 3 weightpercent ruthenium metal and a sample (IA) prepared according to ExampleI, were each separately diluted with a blank alumina support to a finalvolumetric ratio of 3:1 placed in a reactor and purged with nitrogen ata flow rate of 300 ml/min. The reactor was then heated to a temperatureof 80° C. where the feed gas mixture shown in Table 2 was thenintroduced over the catalyst at a flow rate equivalent to a gas hourlyspace velocity (GHSV) of 5,000, 10,000, and 15,000 v/v hr⁻¹.

TABLE 2 Feed Gas Over PREFOX Catalyst, Mol-% CO₂ 11.98 CO 0.47 CH₄ 2.96Ne 5.05 H₂ 46.84 N₂ 4.02 O₂ 1.06 H₂O 27.62 Total 100.00

The reactor was heated to a temperature of about 108° C. This conditionwas maintained for a period of approximately 45 minutes while theeffluent levels of CO, CO₂, CH₄, and O₂ were monitored. The temperaturewas lowered to 90° C. and the catalyst was maintained at thistemperature for approximately 120 minutes. The catalysts of ExamplesI-IV (with pH adjustment) and V (with no pH adjustment) were thencompared under these conditions for their ability to convert CO to CO₂at space velocities ranging from about 5,000 hr⁻¹ to about 15,000 hr⁻¹and the results are shown in Table 3. The results clearly show theadvantage of the pH adjustment in terms of CO conversion for thecatalyst of Examples I-IV compared to the conventionally preparedcatalyst of Example V. As space velocity was increases from about 5,000hr⁻¹ to about 15,000 hr⁻¹, the CO conversion dropped off significantlyrelative to the pH adjusted Examples I-IV.

TABLE 3 Results with 3% Ru on Alumina Base O₂ mol O₂ mol O₂ molSelectivity CO Selectivity Selectivity CO Conversion to CO₂ Conversionto CO₂ CO Conversion to CO₂ Example (90° C.) (90° C.) (90° C.) (90° C.)(90° C.) (90° C.) (adjusted pH) 5,000 hr⁻¹ 5,000 hr⁻¹ 10,000 hr⁻¹ 10,000hr⁻¹ 15,000 hr⁻¹ 15,000 hr⁻¹ I (4.5) 100 26.1 99.9 26.1 99.4 26.6 II(3.0) 99.9 26.5 89.1 47.8 III (2.0) 98.1 26.6 64.3 58.2 49.7 72.7 IV(1.1) 86.0 32.0 54.0 60.7 47.0 69.0 V (none) 43.1 53.4 12.3 57.1 6.2 50

Example VII

The catalyst materials prepared in Examples I-V were analyzed byscanning electron microscopy (SEM) and the ruthenium composition wasdetermined within a spherical particle by energy dispersive analysis byx-rays (EDX). SEM and EDX measurements were made across the interior ofthe particles to determine the ruthenium distribution for the variationin the pH of the ruthenium contacting. Table 4 presents the results ofthis analysis as the ratio of the ruthenium metal content at any pointin the particle to the average ruthenium metal content of the particleacross the radius of the particle from the surface to the center of theparticle. With no pH adjustment, the ruthenium was distributedessentially uniformly throughout the particle (shown as a pH of 0.0). Itwas surprisingly discovered that as the acidity of the rutheniumcontacting solution was adjusted to increase the pH from about 1.0 toabout 4.5, the performance of the prefox catalyst improved. Alsoobserved was as the pH of the ruthenium contacting solution wasincreased from about 1.0 to about 4.5, the ruthenium dispersion changedfrom a uniform distribution to a distribution wherein the ruthenium wasdispersed in a band toward the surface of the particle. Comparing theruthenium metal content at a given point in the particle to the averageruthenium metal content of that same particle clearly illustrates thiseffect. For a uniform distribution profile, the ratio of the rutheniummetal content at all points across the particle to the average rutheniummetal content of the particle should be equal to one. This is clearlythe case for the sample prepared using the standard ruthenium contactingsolution in Example V. When the pH of the ruthenium contacting solutionwas adjusted to a value of 1.1 (Example IV), this ratio began toincrease above 1 at a distance of approximately 500 microns from thesurface of the particle. When the pH of the ruthenium contactingsolution was adjusted to a value of 2.2 (Example III), this ratio beganto increase above 1 at a distance of approximately 300 microns from thesurface of the particle indicating a further concentration of rutheniumin a band towards the surface of the particle. When the pH of theruthenium contacting solution was adjusted to about 3.0, the ratioincreased above 1 in a band at a distance of about 200 microns from thesurface of the particle towards the center, and when the pH wasincreased to about 4.5, the ratio increased to above 1 in a band at adistance above 100 microns from the surface center. Thus, when the pHwas adjusted to about 4.5, essentially all of the metal was dispersed ina band about the outer 100 microns of the particle.

TABLE 4 Ruthenium Distribution in Impregnated 1600 Micron SpheresDistance pH 4.5 pH 3.0 pH 2.2 pH 1.1 pH 0.0 from Edge, Ru/Ru Ru/Ru Ru/RuRu/Ru Ru/Ru Microns (avg) (avg) (avg) (avg) (avg) 0 15.74 4.34 3.16 1.971.38 25 9.25 3.12 2.76 1.74 1.01 50 5.49 2.83 2.52 1.57 1.05 75 1.442.68 2.41 1.42 1.02 100 0.23 2.75 2.29 1.37 1.03 150 0.06 2.87 2.13 1.31.05 200 0.02 4.88 2.16 1.2 1.06 300 0.04 0.05 1.65 1.12 1.02 400 0.030.0 0.08 1.08 0.99 500 0.02 0.01 0.05 0.98 0.98 600 0.07 0.03 0.11 0.620.90 700 0.03 0.02 0.06 0.47 0.90 800 0.06 0.04 0.02 0.45 0.91

The FIGURE shows this trend in the transition from uniform rutheniumdispersion to surface dispersion as a function of the penetration of theruthenium in a band extending from the surface of the particle towardsthe center as a percentage of the ruthenium disposed on the particle.Between the surface of the particle and a depth, or width, equivalent toabout 10 percent of the radius of the particle extending from thesurface towards the center of the shaped alumina carrier, and when theruthenium solution was adjusted to a pH of about 1.1 (Example IV), about22 percent of the ruthenium metal was found therein. When the pH of thecontacting solution was about 1.0 at a penetration of about 50 percentfrom the edge or surface of the particle, the ruthenium content in theband extending from the surface towards the center contained more thanabout 60 percent of the ruthenium metal dispersed on the alumina carrierparticle. When the pH of the ruthenium contacting solution was adjustedto about 2.2, about 35 percent of the ruthenium metal was found in aband extending from the surface towards the center of the particlehaving a width of about 10 percent of the distance from the surface tothe center of the shaped alumina carrier. When the pH of the rutheniumcontacting solution was adjusted to about 3.0, the ruthenium metalcontent in a band extending from the surface toward the center withinthe outer 10 percent of the distance from the surface to the center ofthe shaped alumina particle was greater than 42 percent. When the pH ofthe ruthenium contacting solution was adjusted to about 4.5, essentiallyall of the ruthenium, or greater than about 98 percent of the rutheniummetal, was found in a band extending from the surface toward the centerwithin the outer 10 percent of the distance from the surface to thecenter of the shaped alumina particle.

What is claimed is:
 1. A method for preparing a preferential oxidationcatalyst comprising: a) contacting a shaped alumina carrier with asource of ruthenium metal comprising ruthenium nitrosyl nitrate at a pHof between about 1.0 and about 4.5 to provide a ruthenium-containingcomposition having a ruthenium content of between about 0.5 and about 3weight percent ruthenium metal dispersed on a shaped alumina carrier, atleast 60 percent of the ruthenium metal being present in a bandextending from the surface towards the center and having a width ofabout 50 percent of the distance from the surface to the center of theshaped alumina carrier; and b) reducing the ruthenium-containingcomposition.
 2. The method of claim 1 wherein the reduction is carriedout by contacting the ruthenium-containing composition with a reducinggas at a reducing temperature of between about 200° and about 500° C. 3.The method of claim 1 wherein at least about 35 percent of the rutheniummetal is dispersed in the band wherein the band comprises 10 percent ofthe distance from the surface to the center of the shaped aluminacarrier.
 4. The method of claim 1 wherein at least about 40 percent ofthe ruthenium metal is dispersed in the band wherein the band comprises10 percent of the distance from the surface to the center of the shapedalumina carrier.
 5. The method of claim 4 wherein the band comprisesmore than 98 wt-% of the ruthenium metal.
 6. The method of claim 1wherein the shaped alumina carrier is selected from the group consistingof a sphere, an irregularly shaped particle, a cylinder, a coatedmonolith, and combinations thereof.